The project's long-term goal is to understand the effects of otologic pathology on mechanisms underlying human cochlear and brainstem processes. It proposes a mechanism-based approach that will provide (1) critical neuro-patho-physiological information, and (2) a scientific framework for early diagnoses of difficult-to-identify otologic diseases. In this approach, it uses newly-developed non-invasive measures of the auditory brainstem response (ABR) to investigate known and hypothesized pathological changes in the mechanisms underlying specific processes in the peripheral auditory system caused by small acoustic tumors and Meniere's disease. The specific aims are (1) to demonstrate that small (<1 cm) acoustic tumors alter the synchrony and/or amount of neural output to an extent greater than that predicted by audiometric thresholds, and (2) to determine if patients diagnosed with Meniere's disease have abnormal cochlear temporal-related measures consistent with the presence of cochlear hydrops. In addition, a comparative analysis of the results of these studies will determine whether the proposed measures can distinguish between small acoustic tumors and early Meniere's disease. The significance of the proposed work is that it will (1) provide new insight into the effects of otologic pathology on the theoretical and basic scientific mechanisms underlying the human peripheral auditory system, (2) demonstrate that non-invasive physiologic measures guided by a mechanism-based approach can lead to sensitive diagnoses for small acoustic tumors and early Meniere's disease with good specificity, (3) fill the need for non-invasive measures of peripheral auditory system functionality in humans as new therapeutic measures to improve, prevent destruction of, or possibly restore cochlear function are developed, and (4) test the predictive power of currently emerging models of cochlear and brainstem function across the domains of cochlear physiology, neurophysiology, electrophysiology, and pathology.